The invention relates to a method for removing a platform topsides from a substructure by a floatable, ballastable transporter floating in the sea, in which the transporter is possibly moving due to motions of the sea. The invention further relates to a load transfer unit of a floatable, ballastable transporter, for transferring load from a platform topsides to the transporter during removal of the topsides from a substructure, which removal is carried out by ballasting the transporter, locating the transporter underneath the topsides and deballasting the transporter, causing load transfer units of the transporter to contact and lift the topsides off the substructure. The invention also relates to a combination of a load transfer unit and a lift-bracket attached to an underside of the topsides.
An offshore platform consists of a substructure made from steel or concrete and a topsides comprising one or more decks, production equipment and other facilities which are required to exploit a subsea hydrocarbon reserve. Offshore platforms can also be used for other purposes, e.g. injecting water or gas in the reservoirs, or as living quarters for offshore personnel. When their life-time are ended, the offshore platforms should from an environmental point of view be removed.
Normally, due to the great size of an offshore platform, the topsides will have to be removed first, followed by a removal of the substructure. The topsides may be removed by the method of the invention and similar methods, in which a transporter is ballasted, located underneath the topsides and deballasted to thereby mate with the topsides and lift the topsides off the substructure. In this patent application xe2x80x9ctransporterxe2x80x9d shall mean any kind of transporter, including barges and ships, capable of carrying out such a removal of a topsides.
Topsides"" and transporters are heavy structures, and the transfer of the weight of a topsides from the substructure to the transporter is related to various technical problems. One problem is to align several contact points of the transporter with corresponding contact points of the topsides. Another problem is impacts which will normally occur when the transporter contacts the topsides. A further problem is that complete simultaneous and even contact of several contact points are practically difficult to achieve, which may cause an uneven load distribution. In isolation or together these problems may cause damage both to the transporter and the topsides. To solve these problems both the transporter and the topsides may be provided with some kind of load transfer structures.
Many places, like in the North Sea, there is heavy seas, and almost always the transporter will be moving in the sea prior to the mating with the topsides. If no precautions are made, the motions of the transporter strongly worsen the above discussed problems. Elliptical horizontal motions induced by the combined effect of yaw, surge and sway will worsen the problem of aligning the contacts points of the transporter with the contact points of the topsides. Elliptical vertical motions induced by the combined effect of heave, pitch and roll will cause the transporter to repeatingly contact and lose contact with the topsides during the deballasting of the transporter, causing a number of impacts. This will last until the transporter is deballasted so much that the buoyancy of the transporter is big enough to maintain the contact during a downwardly directed heave movement. Rotating motion, known as roll, pitch and yaw, will worsen the problem of uneven contact between several contact points.
When transferring a heavy topsides from a substructure to a transporter when the transporter is moving, it is required first to establish contact between the topsides and the transporter, and then, when the contact is established, carry out the actual transfer of the weight of the topsides and lift the topsides off the substructure. For simplicity the first phase is called the contact phase, while the second phase is called the lift-off phase. During the contact phase, to avoid that the great mass of the moving transporter creates forces which cause damage, the connection between the topsides and the transporter must be flexible. During the lift-off phase, however, to ensure a stable lift-off, the connection between the topsides and the transporter must be stiff.
WO 99/06270 describes a transporter comprising pontoons which define a moonpool with an open side for the location of a substructure, and a topsides load transfer structure above the pontoons. The transporter can be ballasted and then moved into a position in which it is located underneath the topsides and embraces the substructure, i.e. the substructure is in the moonpool. The transporter can then be deballasted, and the topsides load transfer structure contacts the underside of the topsides and lifts the topsides off the substructure. The transporter can then transport the topsides to a receiver, e.g. a construction yard. The load transfer structure of WO 99/06270 is static, and is not able to provide a solution to the problems of transferring the weight of a topsides from the substructure to the transporter if the sea is in motion.
U.S. Pat. No. 4,607,982 discloses an installation of a platform topsides upon a previously installed substructure. The topsides is mounted on a barge positionable between legs of the substructure. The barge can be ballasted and lowered a sufficient distance to allow leg elements of the topsides to contact and mate with leg support elements of the substructure. Impacts between the structures during the mating operation are absorbed by resilient neoprene pads carried by the leg supports. After the platform structure rests upon the leg supports, the barge is removed, and the platform structure is lowered and levelled by draining a select volume of sand from the leg supports. The system involves a complex use of valved tubing and instrumentation to control the levelling and lowering.
U.S. Pat. No. 5,219,451 discloses a barge for locating a topsides on a previously installed substructure. The topsides is located on the barge, and the barge is positioned between legs of the substructure and ballasted until the topsides is in engagement with the substructure. The legs of the substructure have sand jacks and shock-absorbers made from elastomeric members. The weight of the topsides is transferred to the substructure through the shock-absorbers. The sand-jacks are then used to lower the topsides into final steel-to-steel contact with the legs of the substructure. The barge also carries a topsides support structure comprising sand jacks with shock-absorbers. These sand-jacks are used to lower the topsides support structure from the topsides after the topsides have mated with the substructure.
U.S. Pat. No. 6,027,287 describes a load transfer system for placing a load on a barge onto a substructure, comprising a main transfer connector with a plurality of hydraulic jacks and a secondary transfer connector with a hydraulic lifting jack and a sand hopper with an opening valve for rapid flow-out of sand, the sand hopper having a top on which the hydraulic lifting jack is placed.
In the load transfer structures of U.S. Pat. No. 4,607,982 and U.S. Pat. No. 5,219,451 having sand jacks and neoprene pads or other resilient members, these members are shock-absorbers. All the load of the topsides is transferred through the shock-absorbers, and depending of the characteristics of the shock-absorbers, a gradually stiffer connection between the topsides and the transporter is established. The subsequent lowering of the sand jacks is for levelling and lowering the topsides. Thus the transfer of the topsides with these load transfer structures does not provide a contact phase with a flexible connection between the topsides and the transporter, followed by a lift-off phase with a stiff connection between the topsides and the transporter. These load transfer structures will be suitable for calm seas in which the motions of the transporter caused by the six horizontal and vertical orders of freedom are minimal compared to the North Sea summer operating conditions, and which there is no need for a contact phase with a flexible connection followed by a lift-off phase with a stiff connection.
An object of the invention is to provide a method for removing a platform topsides from a substructure by a floatable, ballastable transporter. Which method shall provide a contact phase with a flexible connection between the transporter and the topsides, followed by a lift-off phase with a stiff connection between the transporter and the topsides. A further object is that the method shall provide positive contact between the transporter and the topsides both during the contact phase and the lift-off phase. A further object is to provide a load transfer unit of a floatable, ballastable transporter, which load transfer unit shall be flexible during the contact phase and stiff during the lift-off phase. A further object is to provide a combination of a load transfer unit of a floatable, ballastable transporter and a lift-bracket of a topsides, which shall provide positive contact between the transporter and the topsides both during the contact phase and the lift-off phase.
The objects are achieved by a method, a load transfer unit and a combination of a load transfer unit and a lift-bracket according to the claims.
The invention thus relates to a method for removing a platform topsides from a substructure by a floatable, ballastable transporter floating in the sea, the transporter is possibly moving due to motions of the sea, comprising the following steps:
a) Providing the transporter with load transfer units, each load transfer unit comprising
a drum having a hatch with a releasable hatch-cover forming a bottom of the drum, the drum is supported by the transporter,
a layer of particulate material resting on the hatch-cover,
a compression spring resting on the layer of particulate material,
a load-element which is slideable in the drum and rests on the spring, and which is adapted to contact the topsides.
b) Providing the topsides with lift-brackets adapted to the load-elements.
c) Ballasting the transporter and locating the transporter underneath the topsides with the load-elements of the transporter in alignment with the lift-brackets of the topsides, in a per se known manner.
d) Deballasting the transporter until the load-elements contact the lift-brackets. In heavy seas, due to the elliptical vertical motions of the transporter, this contact may be repeatingly established and lost. The springs provide flexible support for the loads elements, and this repeated establishing and re-establishing of contact will therefore cause only minor impacts which will cause no damage. In this step the springs act as shock-absorbers.
e) Further deballasting the transporter until the springs are partly or fully compressed, the springs thereby maintain upward forces on the load-elements and ensure that contact between the load-elements and the lift-brackets is maintained during possible heave motion of the transporter. This step corresponds to the above discussed contact phase. It is thereby provided a contact phase with a flexible connection with a positive contact between the transporter and the topsides. This flexible connection may be maintained for long time, maybe several hours,
f) Releasing the hatch-covers to suddenly let out the particulate material from the drums, thereby lowering and relieving the springs, and simultaneously further deballasting the transporter, causing the lift-brackets to contact the drums, and causing the topsides to be lifted off the substructure. The springs are thereby totally or essentially deactivated, and the weight of the topsides is transferred to the transporter through the stiff drums. This step provides a change from the flexible connection to a stiff connection between the transporter and the topsides. The change from a flexible to a stiff connection preferably shall take place in seconds, which is made possible by the sudden release of the hatch-cover. The simultaneous deballasting of the transporter provides the lift-off of the topsides from the substructure. This step thereby corresponds to the above discussed lift-off phase.
Alternatively the topsides may have been cut from the substructure as a preparatory operation, i.e, before the arrival of the transporter. The topsides may have been restrained from movement by installing restraining clamps securing the topsides to the substructure. Additional upward force may be induced into the topsides by pumping out water from the transporter in a quantity which induces an additional 1000 to 2000 tonnes of upward force greater than the weight of the topsides. At the lift-off of the topsides from the substructure release mechanisms in the restraining clamps may be activated, which will create a rapid upward lift which will reduce the risk of damage should the topsides contact the substructure due to heave motion.
g) Further deballasting the transporter until a safe distance between the topsides and the substructure is reached, and remove the transporter from the substructure. The transporter is then ballasted, and the transporter can then be brought to a receiver for the transporter, e.g. a shallow draft vessel located inshore in sheltered waters or a pier.
The invention also relates to a load transfer unit as specified in step a), and a combination of a load transfer unit and a lift-bracket as specified in step b).
Preferably in order to ensure a positive contact in lateral directions during the lift-off phase, the upper portion of the drum and the lift-bracket should be adapted to lock relative movement in horizontal directions when the lift-bracket contacts the drum.
The drum may be located on structural steel of the transporter, and welded or bolted to this structural steel. Preferably, however, the drum is supported by the transporter via a hinged link arm, to enable positioning the load-elements in alignment with the lift-brackets, and to allow lateral motion of the transporter during the contact phase.
The compression spring may be a steel spring, either a helical spring or a disc spring, or a spring made from elastomeric material. Preferably the spring is formed by a laminate of a stiff material and an elastomeric material.
The load-element may take various forms, e.g. it can have the shape of a dowel or trunnion, and the lift-bracket can be provided with a corresponding opening.